A power converter is a power processing circuit that converts an input voltage or current source into a specified output voltage or current. In off-line applications wherein power factor correction, total harmonic distortion (THD) reduction and a stable, regulated voltage are desired, the converter often employs a boost converter.
The converter generally includes an electromagnetic interference (EMI) filter, coupled to a source of alternating current (AC) power. A rectifier bridge, coupling the EMI filter to the boost converter, rectifies the AC power to produce an unregulated DC voltage. The boost converter receives the unregulated DC voltage and generates therefrom a controlled DC voltage. A DC/DC converter, coupled to the boost converter, then converts the high DC voltage (e.g., 400 VDC or 800 VDC) to a lower voltage (e.g., 48 VDC or 24 VDC).
A conventional boost converter generally includes an inductor, coupled between an input voltage (e.g., the unregulated DC voltage from the rectifier bridge) and a power switch. The power switch is then coupled in parallel with a rectifying diode and an output capacitor. The output capacitor is usually large to ensure a constant output voltage to a load (e.g., a DC/DC converter). The output voltage (measured at the load) of the boost converter is always greater than the input voltage.
The boost converter operates as follows. When the power switch is conducting, the rectifying diode is reverse-biased, isolating the output capacitor and, therefore, the load. During this period, the input voltage supplies energy to charge the inductor and an inductor current rises. A stored charge in the output capacitor powers the load. When the power switch is not conducting, the inductor current decreases, as energy from both the inductor and the input flows forward through the rectifying diode, charging the output capacitor and powering the load. The output voltage of the boost converter can thus be controlled by varying a duty cycle of the power switch.
For high AC input voltages, in conjunction with the output voltage of the boost converter being greater than the input DC voltage, the output of the conventional boost converter may be too high for commonly available semiconductor devices. A so-called "three-level" boost converter that provides two equal output voltages has been suggested to accommodate semiconductor devices rated for approximately half the total output voltage. The three-level boost converter generally consists of an inductor and two switching circuits (each having a power switch, rectifying diode and output capacitor) coupled in series. By dividing the total output voltage between two outputs, the three-level boost converter reduces maximum voltage stresses across the semiconductor devices. Separate DC/DC converters may then be used with each output. If two DC/DC converters are used, only half of the total output voltage is applied to each DC/DC converter. Switching devices in the DC/DC converters, therefore, can also be rated at half of the total output voltage.
For high input voltages, the three-level boost converter allows the use of lower voltage switching devices and a smaller boost inductor, thus providing better performance than the conventional boost converter. For wide input applications, however, the conversion efficiency of the three-level boost converter is quite poor at a low end of the input range, since the input current must flow through multiple switching devices.
Accordingly, what is needed in the art is a boost converter topology that obtains an improved conversion efficiency over a wide input range.